Eddy current sensor

Abstract

An eddy current sensor ( 1 ) has at least one measuring coil ( 2 ) that can be supplied with an alternating current, and an evaluation circuit ( 3 ). The eddy current sensor allows the temperature influences on the impedance of the measuring coil to be reliably compensated with a simple design and an evaluation circuit. For this purpose, the eddy current sensor has a compensating coil ( 4 ) which can also be supplied with an alternating current and which is arranged closely to the measuring coil, i.e. in thermal contact therewith, in such a way that the electric fields of compensating coil ( 4 ) and measuring coil ( 2 ) are orthogonal to each other. More particularly, the measuring coil ( 2 ) is in an annular form and the compensating coil ( 4 ) is wound around the measuring coil in the form of a torus.

What is claimed is:  
     
       1. Eddy current sensor with an evaluation circuit, with at least one measuring coil, and with at least one compensat ing coil, wherein the measuring coil a nd the compensating coil can be supplied with alternating currenr, and the compensating coil is arranged in direct vicinity of the measuring coil, so that the electromagnetic fields of the compensating coil and measuring coil are orthogonal to each other, wherein the measuring coil has an annular form and the compensating coil is wound around the measuring coil in the form of a torus, so that th e compensating coil is in thermal c ontact with the measu ring coil. 
     
     
       2. Eddy current sensor of claim  1 , wherein the measuring coil is wound around a coil form. 
     
     
       3. Eddy current sensor of claim  2 , wherein the coil form is formed of a dielectric material. 
     
     
       4. Eddy current sensor of claim  2 , wherein the coil form is formed of a ferromagnetic material. 
     
     
       5. Eddy current sensor of claim  1 , wherein the impedance of the measuring coil (Z A =R A +X A ) and the impedance of the compensating coil (Z B =R B +X B ) are essentially the same under identical environmental conditions. 
     
     
       6. Eddy current sensor of claim  1 , wherein the evaluation circuit comprises means for determining the adjusted real part R C  of the impedance of the measuring coil as a difference between the real parts of the impedances of the measuring coil and the compensating coil (R C =R B −R A ), and that the evaluation circuit comprises means for determining the adjusted imaginary part X C  as a difference between the imaginary parts of the impedances of the measuring coil and the compensating coil (X C =X B −X A ). 
     
     
       7. Eddy current sensor of claim  1 , wherein the measuring coil and compensating coil are connected to a source of voltage that supplies orthogonal voltage components, preferably a sine/cosine-wave generator. 
     
     
       8. Eddy current sensor of claim  7 , wherein the evaluation circuit comprises two multipliers each subsequent to the measuring coil and compensating coil , each multiplier receiving the output voltage of the corresponding coil and one of the two voltage components of the source of voltage, so that at the output of the one multiplier it is possible to tap respectively the real part R A  and R B  of the coil impedance, whereas the imaginary part X A  or X B  of the coil impedance can each be tapped at the output of the other multiplier. 
     
     
       9. Eddy current sensor of claim  8 , wherein low-pass filters are connected to the outputs of the multipliers. 
     
     
       10. Eddy current sensor of claim  8 , wherein an adder is connected to respectively two multipliers at whose outputs the real parts or the imaginary parts of the coil impedances can be tapped. 
     
     
       11. Eddy current sensor of claim  1 , wherein the measuring coil and the compensating coil are connected to a bridge circuit. 
     
     
       12. Eddy current sensor of claim  11 , wherein the bridge circuit comprises resistors R 1  and R 2 . 
     
     
       13. Eddy current sensor of claim  1 , wherein the measuring coil and the compensating coil with capacitors C 1  and C 2  form an oscillating circuit. 
     
     
       14. Eddy current sensor of claim  1 , wherein the measuring coil and the compensating coil are operated by a ringing oscillator. 
     
     
       15. Eddy current sensor of claim  1 , wherein the evaluation circuit comprises a preferably programmable microprocessor for evaluating the adjusted real part R C  and the adjusted imaginary part X C  of the impedance of the measuring coil. 
     
     
       16. Method of operating an eddy current sensor of claim  1 , wherein the real part and the imaginary part of the impedances of the measuring coil and compensating coil are each determined, that by subtraction of the real parts and imaginary parts, the adjusted real part 
       
         
           R C =R B −R A    
         
       
       and the adjusted imaginary part 
       
         
           X C =X B −X A    
         
       
       of the impedance of the measuring coil are determined. 
     
     
       17. Method of claim  16  for operating an eddy current sensor, wherein by dividing the adjusted imaginary part and the adjusted real part the phase angle φ of the impedance of the measuring coil is determined from 
       
         
           tanφ=X c /R C .